If you're looking for king size mattress, minimum you're looking at is 3k for anything decent, but even then you have to really look around for the deal. Realistically you are looking at RM5k and above.

Your budget of RM1,200 won't get you anything good in the queen size range.

You may still be able to get something decent if its single mattress for RM1,200.

Keep in mind, your bed is something you spend 30% of your life on. You should put more money into it.